Re: PMTU, MSS and "fragmentation needed" problem with linux?

[Nico Schottelius]

Thanks for the hints Florian, but still there are open questions:

Florian Attenberger [Thu, Apr 07, 2005 at 07:33:09PM +0200]:
> [...]
> # [From the kernel help:
> #
> #    This option adds a `TCPMSS' target, which allows you to alter the
> #    MSS value of TCP SYN packets, to control the maximum size for that
> #    connection (usually limiting it to your outgoing interface's MTU
> #    minus 40).

The MTU is 1460 or 1492 in tested cases. This means on ppp0, TCPMSS
should be 1420 or maximum of 1452. Why do I see 1460 on the interface?

> #    This is used to overcome criminally braindead ISPs or servers which
> #    block ICMP Fragmentation Needed packets. 

That's clear to me, there are sites out there, debatting that problem.
But:

- Should not _my_ Linux-router tell _my_ client that the mtu choosen
  is too big?
- On the tested connections _all_ icmp types were allowed (disabled iptables)
- As far as I can see, the routers between the two hosts
  don't drop the generated icmp-packets

The first of the questions was the reason I choosed this ML, because I
was wondering why my client does not get the DF-Bit-set, but frag-needed
icmp-message from the linux-router.

Or did I unterstand the technique wrong?

Greetings,

Nico

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